1. Field of the Invention
The invention pertains to large, high tonnage injection molding apparatus wherein mold parts are maintained in contiguous relationship during molding and separated to release the molded product, tie rods interconnecting the mold part platens are employed to prevent mold part separation.
2. Description of the Related Art
Injection molding apparatus into which a material in a fluid state is introduced into a mold under high pressure commonly employs separable mold parts relatively movable between open and closed positions. Upon the parts closing, a cavity is defined for receiving the hardenable injected material, and upon the mold parts being opened or pulled apart, the formed product is removed from one of the mold parts.
The fluid material injected into the mold parts is injected under high pressure in order to completely fill the mold cavity, and the internal pressure within the mold cavity tends to separate the mold parts. The greater the area of the cavity, and the larger the part being molded, the greater the forces being imposed upon the mold parts endeavoring to separate them. Commonly, injection molding apparatus includes spaced platens capable of relative movement toward and away from each other, and a mold part is mounted upon each platen. When the platens are separated, the molded product may be removed from the open mold, and when the mold parts are closed after the platens are moved to their closed position, the molding operation occurs. Tie rods interconnecting the platens are commonly employed to resist the forces tending to separate the mold parts and platens during molding. Commonly, the tie rods are in the form of large bolts, usually threaded at one or both ends.
As the tie rods must bridge the space between the platens, the tie rods are located adjacent the mold parts, and in some instances the tie rods, in effect, surround the mold parts. Accordingly, the tie rods interfere with access to the mold parts and the mold part cavity when removing the formed product, and with large formed products, it is often necessary to manipulate the product once removed from the mold part in order to remove the formed product from the confines of the apparatus.
With injection molding apparatus, the larger the size of the mold parts and cavity, the greater the molding pressures, and a greater number of tie rods are required. Hence, considerable difficulty is encountered in removing large injection formed products from large capacity injection molding apparatus due to the presence of the tie rods, and the tie rods limit the dimensions of products which may be successfully formed by injection molding.
Further, the presence of the tie rods causes considerable difficulty when changing dies and mold parts, and it is necessary to either temporarily remove tie rods from the injection molding apparatus, or space the tie rods sufficiently from each other to permit the heavy mold parts to be properly positioned on the platens.
Access to the interior of the platens for mounting the mold parts thereon has been improved by temporarily releasing one end of the tie rods from a platen, and such molding apparatus is shown in U.S. Pat. Nos. 4,172,872; 4,948,358 and 4,984,980. However, such molding apparatus is not of such construction as to quickly clear the tie rods from the molding parts and provide full access to both of the mold parts.
It has also been known to use hydraulic forces to produce tension forces within the tie rods to improve mold clamping pressures as shown in U.S. Pat. Nos. 4,948,358; 4,984,980 and 5,066,217, but in these patents the tie rods do not sufficiently clear the mold part area to provide the combination of high mold clamping forces and mold part access required, especially when forming large size products.
Access to injection mold part cavities has been improved by developing apparatus permitting the platens to pivot from a mold part alignment relationship after the molding has been completed, and such devices also simplify the mounting of the die or mold parts on the associated platen. Such devices are shown in U.S. Pat. Nos. 4,708,625 and 5,033,955. Apparatus of this type utilizes tension bars and frames to resist the mold pressures, but this type of apparatus does not locate the mold clamping forces in the close proximity to the mold parts during molding as is necessary to produce high quality molded products with a minimum of flashing and very accurate mold part alignment.